Integrated electrical circuits and other microstructured components are usually produced by virtue of a plurality of structured layers being applied onto a suitable substrate, which is usually a silicon wafer. For the purposes of structuring the layers, these are initially covered by a photoresist (resist), which is sensitive to light from a specific wavelength range, e.g. light in the deep ultraviolet (DUV), vacuum ultraviolet (VUV) or extreme ultraviolet (EUV) spectral range. Subsequently, the wafer coated thus is exposed in a projection exposure apparatus. Here, a pattern of diffractive structures, which is arranged on a mask, is imaged onto the resist with the aid of a projection lens. Since the absolute value of the imaging scale generally is less than 1 in this case, such projection lenses are sometimes also referred to as reduction lenses.
The wafer is subjected to an etching process after developing the resist, as a result of which the layer is structured in accordance with the pattern on the mask. The resist which still remained is then removed from the remaining parts of the layer. This process is repeated so often until all layers are applied to the wafer.
The prior art has disclosed illumination systems which use mirror arrays in order to be able to variably illuminate the pupil plane of the illumination system. Examples thereof are found in EP 1 262 836 A1, US 2006/0087634 A1, US 2010/0060873 A1, US 2010/0277708 A1, U.S. Pat. No. 7,061,582 B2, WO 2005/026843 A2 and WO 2010/006687 A1. In general, these are mirror arrays in this case, in which the mirrors can be tilted continuously over a certain angle range.
WO 2012/100791 A1 has disclosed an illumination system which additionally includes a digitally switchable micromirror array. This micromirror array is imaged onto the light-entrance facets of an optical integrator with the aid of a lens. A similar, but differently driven illumination system is known from the European patent application filed on Nov. 22, 2013 and having the application number EP 13194135.3, entitled “Illumination System of a Microlithographic Projection Exposure Apparatus”, the content of which is incorporated by reference in the subject matter of the present application.
EP 2 202 580 A1 discloses an illumination system including a micromirror array, but the latter is not digitally switchable between two switching positions, but rather is continuously tiltable over a relatively large tilting angle range. The micromirror array is used for setting the illumination setting and is synchronized with the light source such that a change of the illumination setting can take place between two light pulses.
Time division multiplexing operation of two light sources is known from U.S. Pat. No. 5,880,817 and US 2007/0181834 A1.